1. Field of the Invention
This invention relates to methods for forming etch masks on substrates which are too large to efficiently employ photolithography techniques. Such etch masks may be used to form such structures as micropoint cathode emitters for field emission flat panel video displays, spacers for liquid crystal displays, quantum dots, or other features which may be randomly distributed on a surface.
2. State of the Art
For considerably more than half a century, the cathode ray tube (CRT) has been the principal device for electronically displaying visual information. Although CRTs have been endowed during that period with remarkable display characteristics in the areas of color, brightness, contrast and resolution, they have remained relatively bulky and power hungry. The advent of portable computers has created intense demand for displays which are lightweight, compact, and power efficient. Although liquid crystal displays (LCDs) are now used almost universally for laptop computers, contrast is poor in comparison to CRTs, only a limited range of viewing angles is possible, and battery life is still measured in hours rather than days. Power consumption for computers having a color LCD is even greater, and thus, operational times are shorter still, unless a heavier battery pack is incorporated into those machines. In addition, color screens tend to be far more costly than CRTs of equal screen size.
As a result of the drawbacks of liquid crystal display technology, field emission display technology has been receiving increasing attention by the industry. Flat panel displays utilizing such technology employ a matrix-addressable array of cold, pointed, field emission cathodes in combination with a luminescent phosphor screen.
Somewhat analogous to a cathode ray tube, individual field emission structures are sometimes referred to as vacuum microelectronic triodes. Each triode has the following elements: a cathode (emitter tip), a grid (also referred to as the gate), and an anode (typically, the phosphor-coated element to which emitted electrons are directed). The cathode and grid elements are generally located on a baseplate, while the anode elements are located on a transparent screen, or faceplate. The baseplate and faceplate are spaced apart from one another. As the space between the baseplate and faceplate must be evacuated, a hermetic seal joins the peripheral edges of the baseplate to those of the faceplate.
Although the phenomenon of field emission was discovered in the 1950's, it has been within only the last ten years that extensive research and development have been directed at commercializing the technology. As of this date, low-power, high-resolution, high-contrast, monochrome flat panel displays with a diagonal measurement of about 15 centimeters have been manufactured using field emission cathode array technology. Although useful for such applications as viewfinder displays in video cameras, their small size makes them unsuited for use as computer display screens.
Several engineering obstacles must be overcome before large screen field emission video displays become commercially viable. One such problem relates to the formation of load-bearing spacers which are required to maintain physical separation of the baseplate and the phosphor coated faceplate in the presence of external atmospheric pressure. Another problem relates to masking the baseplate in order to form the emitter tips. When the baseplate is no larger than the semiconductor wafers typically used for integrated circuit manufacture, the process disclosed in U.S. Pat. No. 5,391,259 to David Cathey, et al. works splendidly, as the mask particles can be formed from photoresist resin using a conventional photolithography process. However, when the baseplate is larger than those semiconductor wafers, conventional photolithographic techniques utilized in the integrated circuit manufacturing industry are much more difficult to apply. This disclosure is directed toward the problem of forming emitter tips on a large area baseplate.
Erie Knappenberger of Micron Display Technology, Inc. has proposed a new method for forming a mask pattern on a field emission display baseplate using beads or particles as the masking medium. As etch masks for a random pattern of similarly sized dots formed by dispensing glass or plastic beads suspended in a solution on an etchable surface are known to suffer from the problem of aggregation (i.e., multiple beads aggregating together on the surface), a nebulizer or atomizer is used to generate an aerosol containing particles. A monodispersed aerosol may be produced by utilizing a nebulizer or atomizer which produces droplets which are less than twice the size of the beads or particles within the mixture that is to be atomized. Alternatively, the mixture may be diluted so that the probability of two particles or beads being included within a single droplet is small. The aerosol thus created is then applied to a substrate, producing a uniform monolayer of particles having substantially no aggregation. The particles may be used as a micropoint mask pattern which, when subjected to an etch step, forms field emitter tips for a field emission display or other micro-type structures. An alternative method for minimizing aggregation is to use two types of particles, one of which functions as a masking particle, the other which functions as a spacer particle. Thus, even if aggregation of particles is intentionally generated, the spacer particles may be removed by various techniques such as a chemical dissolution or evaporation, thereby minimizing aggregation of the masking particles themselves.
Another masking technique taught by U.S. Pat. No. 5,676,853 to James J. Alwan, utilizes a mixture of mask particles and spacer particles. The spacer particles space the mask particles apart from one another, and the ratio of spacer particle size to mask particle size and the ratio of spacer particle quantity to mask particle quantity control the distance between mask particles and the uniformity of distribution of mask particles.
An additional masking technique taught by U.S. Pat. No. 5,510,156 to Yang Zhao utilizes latex spheres which are deposited in a monolayer on a surface, shrunk to reduce their diameters, and subsequently covered with an aluminum layer. When the reduced-diameter spheres are dissolved, apertures are formed in the aluminum layer, and the apertures are subsequently utilized to etch an underlying layer.
Still another masking technique is taught by U.S. Pat. No. 5,399,238 to Nalin Kumar. This technique relies on physical vapor deposition to place randomly distributed metal nuclei on a surface. The nuclei form a discontinuous etch mask on the surface of a layer to be etched.
Even under the best of circumstances, the use of the foregoing masking techniques will produce totally random patterns.
A more regular mosaic pattern may be produced by the process disclosed in U.S. Pat. No. 4,407,695 to Harry W. Deckman. Using this process, a monolayer film of spherical colloidal particles is deposited on a surface to be etched. A spinning step which applies centripetal force to the particles is employed to improve packing density. The packed monolayer is then ion etched to produce tapered columnar features. The tapering of the features results from continuing degradation of the colloidal particles during the ion etch step.
A masking technique similar to that patented by Deckman is disclosed in U.S. Pat. Nos. 5,220,725; 5,245,248 and 5,660,570 to Chung Chan, et al. This technique is disclosed in the context of fabricating an interconnection device having atomically sharp projections which can function as field emitters at voltages compatible with conventional integrated circuit structures. The projections are formed by creating a monolayer of latex microspheres on a surface to be etched by spraying or pouring a colloidal suspension of the microspheres on the surface and, then, subjecting the monolayer covered surface to either a wet etch or a reactive-ion etch.
What is needed is a simplified process for forming more regular mask patterns having no masking defects caused by two or more masking particles being too close to one another. The desired process should be capable of producing mask patterns which suffer little or no degradation during plasma etches. In addition, the process should be capable of forming masks which are usable for both reactive-ion etches and wet etches.